Note: Descriptions are shown in the official language in which they were submitted.
- 2 ~2~7~
BACKGROUND OF THE INVENTION
The present invention relates to a filter med-
ium in the form of a hollow fiber, and more particularly
to a hollow fiber filter medium having a large pore dia-
meter made of a polysulfone resin and a process for thepreparation thereof.
Hollow fiber filter mediums have the advantages
that a filter is made up in a small size with a simple
structure and the flow of a fluid becomes uniform, since
the effective membrane area per unit volume can be made
large as compared with flat filter mediums. Therefore,
in recent years, porous hollow fibers have been utilized
as filter mediums in various fields in place of the flat
filter mediums.
Among the known hollow fiber filter mediums,
hollow fibers made from resins such as polyvinyl alcohol,
cellulose acetate, polymethyl methacrylate, polypropylene
and polyethylene are known as hollow fiber filter mediums
having a large pore diameter. The hollow fiber filter
mediums made from these resins are not always satisfac-
tory in points of the performance and preparation method
thereof. For :instance, they have the defects in perfor-
mance that steam sterilization requiring high ~empera-
tures is impossible because of lack in heat resistance,
the rate of filtration is low and the chemical resistance,
mechanical strength or affinity for a living body is in-
sufficient. Also, the methods for the preparation there-
of have the defects that because of using high molecular
weight compounds as pore-forming agents, a long time is
required for removing them by extraction and :it is also
necessary therefor to use toxic soLverlts.
~ polysulfone resin is superior in heat resis-
tance, mechanical resistance, chemical resistance and af-
finity for a living body. It has been utilized as a sup~
port for ultrafiltration membrane and reverse osmosis
membrane, and many literature references therefor have
also been published. However, a polysulfone hollow fiber
filter medium having a large pore diameter, at least hav-
ing pores or openings of 0.01 to several microns in dia-
q~
7~
meter in the inner and outer surfaces, cannot be preparedby a conventional technique, and no polysulfone hollow
fiber filter medium having pores or openings of not less
than 0.01 ~m. in diameter a-t least in the inner and outer
surfaces has been provided up to now. For instance, pro-
cesses for preparing polysulfone hollow fibers are reported
in Japanese Unexamined Patent Publications (Tokkyo Kokai)
Nos. 16378/1979 dated February 6, 1979, 143777/1979 dated
November 9, 1979 and 145379/1979 dated November 13, 1979,
all in the name of Asahikasei Kogyo Kabushiki Kaisha, and
152704/1981 dated November 26, 1981 and 82515/1982 dated
May 24, 1982, both in the name of Kanegafuchi Kagaku Kogyo
Kabushiki Kaisha, and by Cabasso et al. in the Journal of
Applied Polymer Science, Vol. 20, 2377-2394 ~1976), Vol.
21, 165-180 (1977) and Vol. 21, 1883-1900 (1977). These
publications disclose processes for preparing hollow fi-
bers having the so-called asymmetric structure, that is,
hollow fibers substantially having a thin, dense layer
containing pores of less than 0.01 ~m. in diameter in ei-
ther one or both of the inner surface and the outer sur-
face thereof. The amount of water permeation of these
hollow fibers is small, even if the dense layer is made
as thin as possible.
It is an object of the present invention to pro-
vide a porous hollow fiber having a large pore diameterand useful as a filter medium.
A further object of the invention is to provide
a hollow fiber filter medium having a large pore diameter
and excellent heat resistance, mechanical strength, chem-
ical resistance and affini-ty for a living body.
Another object of the LnventLon is to provide a
process for preparing a porous hollow fiber useful as a
filter medium.
These and other objects of the presen-t invention
will become apparent from the description hereinafter.
SUMMARY OF THE INV_NTION
In accordance with the present invention, there
is provided a filter medium in the form of a porous
.,
~23785~3
hollow fiber made of a polysulfone having a network
structure over the entire thickness from the inner surface
to the outer surface, the pores having a maximum diameter
of 0.1 to 5 ~m., the openings of the pores formed in the
5 inner surface having a maximum diameter of 0.01 to 10 ~m.
and the openings of the pores formed in the outer surface
having a miximum diameter of 0.01 to 5 ~m.
The polysulfone hollow fiber filter medium of
the present invention has excellent heat resistance,
mechanical strength, chemical resistance and affinity for
a living body. Aslo, since the pores formed by the
network structure have a maximum diameter of 0.1 to 5 ~m.,
the openings formed in the inner surface of the hollow
fiber have a maximum diameter of 0.01 to 10 ~m. and the
openings formed in the outer surface of the hollow
fiber have a maximum diameter of 0.01 to 5 ~m., the
hollow fiber filter medium of the invention has an
excellent water permeability (filtration ability).
BRIEF DESCRIPTION OF THE DRAWINGS
Figs. 1 to 10 are microphotographs of the
hollow fibers of the present invention, in which Fig. 1
is a photograph of the section of the hollow fiber of the
invention observed by a microscope of 100 magnifications;
Fig. 2 is a photograph of the inner surface of the hollow
fiber shown in Fig. 1, observed by a microscope of 11,100
magnifications Fig. 3 is a photograph of the outer
surface of the hollow fiber shown in Fig. 1, observed
by a microscope of 30,000 magnifications; Fig. 4 is a
photograph of the section of another hollow fiber of the
invention observed by a microscope of 100 magnifications
Fig. 5 is a photograph of the outer surface area in the
sectional portion enclosed with the rectangle shown in
Fig. 4, observed by a microscope of 1,000 magnifications
Fig. 6 is a photograph of the inner surface of the hollow
fiber shown in Fig. 4, observed by a microscope of 5,000
magnifications Fig. 7 is a photograph of the outer
surface of the hollow fiber shown in Fig. 4, observed by
1~3~ 9
-- 5
a microscope of 10,000 magnifications; Fig. 8 is a
pho-tograph of the inner surface area in the section of
the hollow fiber shown in Fig. 4, observed by a
microscope of 10,000 magnifications; Fig. 9 is a
photograph of the middle portion between the inner and
outer surface areas in the section of the hollow fiber
shown in Fig. 4, observed by a microscope of 10,000
magnifications; and Fig. 10 is a photograph showing the
outer surface and the outer surface area in the section
of the hollow fiber shown in Fig. 4, observed by a
microscope of 10,000 magnifications
Fig. ll is a graph showing the relationship
between the temperature and the viscosity of a spinning
solution (13 ~ by weight solution of a polysulfone in a
mixture of N-methyl-2-pyrrolidone and propylene glycol)
as used in the preparation of the hollow fiber of the
invention; and
Fig. 12 is a graph showing the relationship
between the proportion of the propylene glycol non-solvent
in a mixed solvent used for the preparation of a spinning
solution and a transition temperature at which the
viscosity of the spinning solution becomes maximum.
DETAILED DESCRIPTION
The hollow fiber filter medium of the present
invention is prepared from a polysulfone. Any of known
polysulfone resins can be employed in the present
invention, and aromatic polysulfone resins are preferable.
Representative examples oE the polysuflone resins used in
the invention are a polysulfone havlng recurring units of
the general formula ~
~ O ~ C ~ O- ~ -52 ~ (I)
and a polysulfone having recurring units of the general
formula (II):
1237859
-- 6
~ 2 ~ (II)
The polysulfone (I) which has hitherto been used as a
material of ultrafiltration membranes, is excellent in
fundamental characteristics such as heat resistance,
mechanical strength, chemical resistance and affinity for
a living body, and accordingly is preferably used as a
material for preparing the filter medium of the present
invention.
The filter medium of the present invention is
in the form of a hollow fiber having a network structure
or a sponge structure over the entire thickness from the
inner surface to the outer surface. The maximum diameter
of the pores of the porous hollow fiber is from 0.1 to
5 ~m., the maximum diameter of the pore openings formed
in the inner surface of the hollow fiber is from 0.01 to
10 ~m., and the maximum diameter of the pore openings
formed in the outer surface is from 0.01 to 5 ~m. The
filter medium of the invention has a water permeability
of not less than 2.0 X 10 3 g./cm.mmHg.min.
The term "maximum diameterl' as used herein
means the length of the largest minor axis of the pores
or openings of the hollow fiber observed by a microscope.
The structure of the filter medium of the
present invention will be explained with reference to
the drawings. Fig. 1 which is a micrograph of the section
of the hollow fiber of the invention, shows that the
hollow fiber has a uniform network or sponge structure
and the maximum diameter of the pores is about 5 ~m.
Figs. 2 and 3 show that openings of the pores are present
on the inner wall surface and the outer wall surface of
the hollow fiber and the maximum diameters of the openings
are about 0.04 ~m. and about 0.3 ~m., respectively.
Figs. 4 and 5 which are micrographs of the section of
another hollow fiber of the invention, show that the
hollow fiber has a uniform network or sponge structure
and the maximum diameter of the pores is about 1.5 ~m.
1237~
-- 7
Figs. 6 and 7 show that openings of the pores are present
on the inner wall surface and the outer wall surface of
the hollow fiber and the Maximum diameters of the openings
are about 1 ~m. and about 1.5 ~m., respectively. Fig. 8
in which the portion near the inner wall surface in the
section of the hollow fiber in Fig. 4 is enlarged, and
Fig. 6 show the state that the network or sponge structure
forms openings of indeterminate shape on the inner wall
surface with a wide distribution of opening diameter.
Also, Fig. 10 in which the outer wall surface and the
portion near the outer wall surface in the section of the
hollow fiber shown in Fig. 4 are enlarged, shows the
state that the network or sponge structure forms elliptic
or circular openings on the outer wall surface.
As is clear from the drawings and the
explanation therefor, the filter medium of the present
invention has the following feature.
(1) The portion between the inner surface and the outer
surface of the hollow fiber has a network or sponge
structure.
(2) A dense layer having only smaller pores than pores
present in the sponge, namely a skin layer, is not
present in the inner and outer surface areas of the
hollow fiber.
(3) The openings in the inner and outer surfaces of
the hollow fiber are those formed by a part of the
sponge broken outwardly.
(4) The openings formed in the inner surface are in an
indeterminate form and have a wide size distribution
within the range of not more t:han the maximum
diameter, and the rate of the area of openings in
the inner surface is large.
(5) The water permeability is large, and it is possible
to obtain the filter medium having a water
permeability of 2.0 X 10 3 to 40 X 10 3
g./cm.2mmHg.minute.
(6) The openings formed in the outer surface are elliptic
or circular.
~3~
-- 8
The above features indicate that the hollow
fiber filter medium of the present invention is greatly
different from conventional polysulfone hollow fibers.
The inner diameter and wall thickness of the
hollow fiber filter medium of the invention are not
technically limited to particular ranges. Usually, the
inner diameter is selected from 100 ~m. to 3 mm. and the
wall thickness is selected from 20 to 500 ~m. according
to the purposes.
When either one of the maximum diameters of
the openings on the inner surface and the openings on the
outer surface is less than 0.01 ~m., the amount of water
permeated is smaller than that of a conventional
ultrafiltration membrane, and accordingly one of the
advantages of the invention, namely a good water
permeability J iS lost. Of course 7 another feature of the
invention that relatively large particles can permeate
through the filter medium is also lost. On the other
hand, when the maximum diameters of all of the pores in
the sponge body and the openings in the inner and outer
surfaces exceed 5 ~m., 10 ~m. and 5 ~m., respectively,
the mechanical strength of the hollow fiber is lowered.
Accordingly, it is preferable in practical use that the
maximum diameters of the openings in the inner surface,
the pores and the openings in the outer surface are from
0.01 to 10 ~m., from 0.1 to 5 ~m. and from 0.01 to 5 ~m.,
respectively. It is preferable that the diameter of the
pores is uniform over the entire thickness of the hollow
fiber, but a certain distribution in pore diameter is
permissible so long as the difference is not so large.
The usual wet process for preparing a hollow
fiber is applicable to the preparation of the hollow
fiber filter medium of the present invention. For
instance, the hollow fiber is prepared by extruding a
spinning solution, namely a solution of a polysulfone
in an organic solvent, from a slit of a ring nozzle
together with an inside coagulating liquid which is sent
forth from the inside portion of the nozzle, and
_ 9 - ~ ~3~
immediately or after passing through a dry space of not
more than 50 cm. length, preferably not more than 20 cm.
length, from the nozzle, bringing the whole into contact
with an outside coagulating liquid. In the present in-
vention, it is essential that the spinning solution of a
polysulfone have a transition temperature (Tc) at which
the viscosity rise changes to the viscosity drop when the
temperature of the spinning solution is dropped, and that
the spinning solution which is formulated to have the
transition temperature Tc and maintained at a temperature
of not less than Tc is extruded from the ring nozzle to
the outside coagulating liquid, while maintaining at least
one of the inside coagulating liquid, the gas contacting
the extrudate in the dry space (of not more than 50 cm.
length, preferably not more than 20 cm. length) and the
outside coagulating liquid at a temperature of less than
Tc, whereby a polysulfone hollow fiber having a network
or sponge structure over the entire thickness extending
from the inner surface to the outer surface of the hollow
fiber is obtained.
The spinning solution is prepared by dissolving
a polysulfone in a specific solvent~ For obtaining the
spinning solution having a desired transition temperature
Tc, it is desirable to use a solvent having a relatively
high boiling temperature. Preferable solvents used for
preparing the spinning solution are, for instance, di-
methyl sulfoxide and mixed solvents of a major amount
of one or more kinds of good solvents for polysulfone
resins such as N-methyl-2-pyrrolidone, dimethylEormamide
and dimethylacetoamide and a minor amount of one or more
kinds of non-solvents for polysulfone resins, which are
used for adjusting the transition temperature Tc, e.g.
polyhydric alcohols such as glycerol, propylene glycol,
ethylene glycol and butanediol and alcohols having a
high boiling temperature such as cyclohexanol. The use
of the mixed solvents is preferred, since a spinning
solution having a desired transition temperature is
obtained. The concentration of the polysulfone in the
~L~3~ 3
-- 10
spinning solution is from 8 to 25 % by weight, preferably
10 to 17 ~ by weight. When the concentration is less
than 8 % by weight, the viscosity of the spinning
solution is low and the spinning operation is difficult.
When the concentration is more than 25 ~ by weight, it
is di~ficult to prepare a hollow fiber having large
pores and openings.
The transition temperature of the spinning
solution is from 30 to 150 C., preferably 50 to 150C.
When the transition temperature is less than 30C., it is
necessary to use a cooler to maintain at least one of the
inside coagulating liquid, the outside coagulating liquid
and the gas present in the dry space, below the
transition temperature. When the transition temperature
is more than 150C., the viscosity of the spinning
solution becomes low because the spinning solution must
be maintained at a high temperature, and accordingly
the spinning is difficult.
In many cases, the transition temperature
Tc has a close relation to the cloud point of the
spinning solution, and is related to the phase separation
due to the temperature drop of the spinning solution.
Fig. 11 is a graph showing the relationship between the
temperature and the viscosity of a solution, as used in
the process of the present invention as a spinning
solution, consisting of 13.0 % by weight of a polysulfone
(commercially available under the commercial name "Udel
Polysulfone P-3500" made by Union Carbide Corp.), 26.1
by we;ght of propylene glycol and 60.9 ~ by weight of
N-methyl-2-pyrrolidone (hereinafter reEerred to as "NMP").
Fig. 12 is a graph showing the change in the transition
temperature Tc with the change in the proportion of
propylene glycol (PG) in a PG-NMP mixed solvent of the
13.0 ~ by weight polysulfone solution shown in Fig. 11.
As shown in Fig. 12, the transition temperature Tc can
be controlled by changing the composition of a mixed
solvent. The solvents used in the present invention
serve as good solvents at temperatures of not less than
* Trade Mark
~ ,,
;i9
Tc. Solutions of polysulfone having a composition such
that the viscosity thereof suddenly drops with the
temperature drop in the temperature range of less than
Tc are suited for use in the process of the invention as
spinning solutions.
Non-solvents for the polysulfone having a
miscibility with the solvents for the polysulfone
and having an action of coagulating the polysulfone with
contacting the spinning solution are used as inside
coagulating liquids and outside coagulating liquids in
the present invention. Examples of the coagulating
liquids used in the invention are, for instance, water,
mixtures of the above-mentioned good solvents for the
polysulfone and water, and alcohols such as methanol,
ethanol and isopropanol.
In the present invention, a hollow fiber is
formed by extruding the spinning solution maintained at
a temperature of not less than the transition temperature
Tc together with the inside coagulating liquid from a
ring nozzle to a coagulation bath. The inner diameter
and wall thickness of the hollow fiber of course vary
depending on the size of the nozzle, the draft ratio and
the inflation ratio. However, the draft ratio is within
the range of about 0.8 to about 3 and the inflation ratio
is within the range of about 0.4 to about 1.3, and
accordingly they do not exert a large influence upon
the structures of the inner surface, the network and the
outer surface. The structures of the inner surface, the
network and the outer surface can be properly adjusted
by changing other spinning conditions such as the
temperature of the inside coagulating liquid. The draft
ratio and the inflation ratio are defined as follows:
V ( dl - d 22 ) ~r
Draft ratio =
4q2
Inflation ratio = 2
Vd
- 12 - 1~37~59
wherein dl is an outer diameter of the ring nozzle, d2
is an inner diameter of the ring nozzle, ql is an amount
of spinning solution sent forth, q2 is an amount of in-
side coagulating liquid sent forth, and V is a rate of
spinning. In a conventional process for preparing a
polysulfone hollow fiber, the hollow fiber is broken or
burst during the spinning unless both the draft ratio
and the inflation ratio are adjusted about 1. In this
respect, the process of the invention has a wide allow-
able range. A ring nozzle is usually employed as a noz-
zle from which the spinning solution is sent forth. The
spinning rate is determined according to the above men-
tioned equations so as to satisfy the draft ratio and
the inflation ratio with giving consideration to a rela-
tion to other factors. When the spinning rate is sev-
eral tens of meters per minute, the operation is easy.
The spinning solution is sent forth from the
ring nozzle together with the inside coagulating liquid
which is sent forth into the inside of the tubular ex-
trudate, and immediately or after passing through a dryspace of not more than about 50 cm. length, preferably
not more than about 20 cm. length, they are brought into
contact with the outside coagulating liquid. At that
time, at least one of the inside coagulating liquid, the
gas in the dry space and the outside coagulating liquid
is maintained at a temperature of less than the transi-
tion temperature Tc of the spinning solution. When the
length of the dry space, in other words, the distance
between the nozzle tip and the outside coagulating li-
quid, is less than several centimeters, :Lt :is prcferableto maintain both of the inside and outs:ide coagulating
liquids at a temperature of less than tC, since it is
difficult to proceed with the coagulation in the dry
space. When the dry space is more than several centime-
ters, it is relatively easy to proceed with the coagula-
tion in the dry space and, therefore, both temperatures
of the inside and outside coagulating liquids may not be
less than Tc and, in a preferable embodiment, the gas in
the dry space
lZ~37~5~
- 13
is maintained at a temperature of less than Tc. More
preferably, all of the inside and outside coagulating
liquids and the gas are maintained at a temperature of
less than Tc. Also, preferably, the coagulation is
substantially completed in the dry space. It is not
necessary to use a specific gas as a gas in the dry space,
and air is sufficient. Also, it is not particularly
required to close the dry space, and the open state is
sufficient. In general, the immersion time of several
seconds in the outside coagulating liquid is sufficient.
It is preferable that the difference between
the transition temperature Tc of the spinning solution
and the lowest temperature among the inside and outside
coagulating liquids and the gas in the dry space is not
less than 10C., preferably not less than 20C.
When the spinning conditions are adequately
set as mentioned above, the spinning solution is suddenly
coagulated by the extraction action and the cooling action
of the coagulating liquids before the flocculation of
polysulfone at the contacting surfaces with the
coagulating liquids sufficiently develops, thus forming
a porous hollow fiber having no skin or dense layer.
Also, when the concentration of the spinning solution is
from about 10 to about 17 % by weight, the porous hollow
fibers of various grades having pores of about 0.01 to
several micrometers in diameter can be easily prepared,
for instance, merely by changing the temperature of the
spinning solution.
The coagulation owing to the coagulating
liquids is usually caused by extraction of a solvent with
the coagulating liquids from the extruded spinning
solution. On the other hand, in the process of the
invention, the coagulation of polysulfone is caused by
simultaneous progress o~ the extraction of a solvent
and cooling of the spinning solution to less than Tc.
Therefore, from the viewpoint of the coagulation action
owing to the above solvent extraction, both of the liquids
having a rapid extraction action and the liquids having a
<s! .v~
237~
mild extraction action can be employed in the present
invention, unlike a conventional process. In a con-
ventional process, in case of using a coagulating li-
quid having the coagulating action owing to rapid sol-
vent extraction, the surface contacting the coagulating
liquid becomes a dense layer having pores of less than
0.01 ~m. in diameter. Also, a relatively uniform sponge
structure is not formed, but the section of the hollow
fiber has -the so-called asyrnmetric structure in which
the pores increasingly become large toward the inside
from the surface. In case of using a coagulating li-
quid having the coagulating action owing to mild sol-
vent extraction, the conventional process produces a
hollow fiber having a dense structure over the entire
wall thickness, and the water permeability is very low.
The reason is that the coagulation of the spinning sol-
ution is made only by extraction of the solvent in the
conventional process.
The hollow fiber immersed and coagulated in
the outside coagulating liquid is subsequently immersed
in water, etc. to remove the solvent remaining in the
fiber, and if necessary, then dried. The drying is car-
ried out preferably at a temperature of less than about
150~C. not causing the deformation of the produced hol-
low fiber.
The hollow fiber filter medium of the present
invention is prepared by a simple process, and has pores
having a large diameter over the entire wall thickness
extending from the inner surface to the outer surface,
thus has a high water permeab:ility r and moreover, is re-
sistant against steam ster:Lli~ation and excellent in me-
chanical strength, chemical resistance and affinity for
a living body. Accordingly, it has great practical value
as a filter for general use or for medical use.
The present invention is more specifically
described and explained by means of the following Ex-
amples, in which all % and parts are by weight unless
. .~
~37~5~
- lS
otherwise noted. It is to be understood that the present
invention is not limited to the Examples, and various
changes and modifications may be made in the invention
without departing from the spirit and scope thereof.
Examples 1 to 10
In 87 parts of a mixed solvent consisting of
29.5 % of propylene glycol and 70.5 ~ of N-methyl-2-
pyrrolidone (NMP) was dissolved 13 parts of a polysulfone
~commercially available under the commercial name "Udel
Polysuphone P-3500" made by Union Carbide Corp.) at
110C. for 3 hours with agitation. The resulting
solution was defoamed under reduced pressure to give a
spinning solution A. The spinning solution A had a
transition temperature of 73 C. The spinning was carried
out by extruding the spinning solution A from a ring
nozzle having an inner diameter of 400 ~m. and an outer
diameter of 600 ~m. at a rate of 3.4 g./minute into an
outside coagulating liquid under the conditions shown in
-0 Table 1, while extruding an inside coagulating liquid
maintained at a prescribed temperature at a rate of 2.5
ml./minute from the inside of the ring nozzle.
The characteristics of the obtained hollow
fibers are shown in Table 2, wherein Di(max) shows the
largest minor axis ~maximum diameter) of the openings
present on the inner wall surface of a hollow fiber,
Dn~max) shows the largest minor axis of the pores present
in the sponge structure in section of the hollow fiber,
and Do~max) shows the largest minor axis o~ the openi~ns
present on the outer wall surface of the hollow fiber,
and they are observed and measured by a microscope.
Examples ll to 14
In 87 parts of a mixed solvent consisting of
28.7 % of propylene glycol and 71.3 % of NMP was
dissolved 13 parts of the same polysulfone as used in
Examples 1 to 10 at 110C. for 3 hours with agitation.
The resulting solution was defoamed under reduced pressure
~3~78~
- 16
to give a spinning solution B having a transition
temperature of 56C. The spinning was carried out by
extruding the spinning so:Lution B from a ring nozzle
having an inner diameter of 350 ~m. and an outer diameter
of 550 ym. at a rate of 3.2 g./minute into an outside
coagulating liquid under the conditions shown in Table 1,
while extruding an inside coagulating liquid maintained
at a prescribed temperature at a rate of 1.8 ml./minute.
The characteristics of the obtained hollow
fibers are shown in Table 2.
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Example 15
A porous hollow fiber having an inner diameter
300 ~m., an outer diameter 360 ~m., a maximum diameter
0.4 ~m. of openings on the inner surface, a maximum dia-
meter 1 ~m. of pores in the section and a maximum diame-
ter 1 ~m. of openings on the outer surface was prepared
in the same manner as in Examples 1 to 10. A bundle of
150 hollow fibers was placed in a polycarbonate pipe hav-
ing an inner diameter 9 mm.~ an outer diameter 13 mm. and
a length 160 mm., and the bundle was potted at both ends
of the pipe. The potted bundle was then cut off at the
ends of the pipe to open the inner surface of each fiber,
and inlet and outlet ports were attached to both ends of
the pipe.
The obtained filter was steam sterilized at
120C. for 30 minutes, and the bubble point and filtra-
tion performance to cow blood of the filter were mea-
sured. Also, with respect to the non-steam sterilized
hollow fiber, the bubble point and filtration perform-
ance to cow blood were measured in the same manner.
No change between the sterilized filter and
the nonsterilized filter was observed, thus it was con-
firmed that the filter medium of the invention is proof
against steam sterilization at high temperatures. Also,
the appearance of the filter was not changed by steam
sterilization.
